The present invention generally relates to a method and an apparatus for drying wafers after a wet bench process and more particularly, relates to a method and an apparatus for drying wafers by alcohol vapor after a wet bench process is conducted by incorporating a DI water rinse step, an inert gas blown dry step and a step of reducing the alcohol vapor flow rate into the drying tank.
In the fabrication of semiconductor devices, a large quantity of deionized (DI) water is frequently used to clean wafers in a wet bench process. For instance, when residual chemical must be removed from the surface of a wafer, DI water rinse is used in the wet bench process to perform major wafer cleaning operations such as quick-dump-rinse and cascade overflow rinse. It is desirable that the surface of the wafer be cleaned by DI water after a chemical or polishing process has been conducted on the wafer, i.e. oxide or nitride deposition, etching or chemical mechanical polishing process. The wet bench wafer cleaning step can be accomplished by equipment that is installed either in-line or in a batch-type process.
A typical automated wafer scrubber combines brush and solution scrubbing by DI water. The scrubber utilizes a hyperbolic high-pressure spray of DI water with a retractable cleaning brush. A typical wafer scrubbing process consists of a DI water spray step followed by a spin dry and nitrogen gas blow dry step. More recently, the solvent drying technology such as the use of isopropyl alcohol (IPA) has been developed to further improve the drying technology.
In a solvent drying technology, such as one that utilizes IPA shown in FIG. 1, the drying process is conducted in a static manner or with the wafer positioned statically without movement. The wafer drying 10 is constructed of a drying tank 12 constructed with a wafer receptacle 14, a chiller 16, a sidewall heater 18 and a bottom heater 20. A cleaned and wet wafer is transported into the drying tank 12, or the vapor chamber. A vapor of IPA is transported into the chamber cavity 22 by a carrier gas such as a high purity nitrogen, or any other high purity inert gas. The vapor enters into cavity 22 is then heated by the bottom heater 20 such that IPA is further vaporized and rises into the cavity 22. The wafer 24 is surrounded by the IPA vapor and, due to the high volatility of IPA, water on the wafer surface can be evaporated away without leaving any water mark, contaminating particles or metal particles. The vapor pressure of IPA can be suitably adjusted such that there is a steady flow of IPA vapor in the cavity 22 fed from the IPA reservoir tank 26.
In the conventional IPA drying tank 10 shown in FIG. 1, the only moving part for transferring wafers into and out of the chamber cavity is a robot arm. There are no other moving parts which can produce contaminating particles. The IPA drying chamber can thus be kept in an extremely clean condition to avoid any contamination of the wafer surface. To further maintain the cleanliness of the chamber cavity 22, an air filter 28 is utilized for filtering incoming air into the cavity 22 and for providing a suitable flow rate of the IPA vapor. After the cleaning process is completed, the water-containing IPA vapor is condensed by the chiller 16 into IPA liquid and is collected at the bottom of the drying chamber 12 for recycling and reuse by the process. The IPA vapor drying process is normally controlled by three major elements, i.e. the purity and the water content of IPA; the flow rate and flow speed of the IPA vapor; and the cleanliness of the IPA vapor.
An improved solvent drying technique has been proposed in recent years which is similar in principal to that described above. In a Maragoni dryer, the drying principal is based on the different surface tensions of IPA and DI water. The different surface tensions cause the ejection of water molecules from the wafer surface which are then collected by a reservoir in the drying apparatus. The Maragoni drying process is carried out by slowly withdrawing a wafer from a DI water tank immersed in DI water. At the same time, IPA vapor carried by N2 carrier gas is flown onto the wet wafer surface such that IPA is saturated on the exposed wafer surface above the water level. Since the concentration of IPA on the surface of the exposed wafer is larger than the concentration of DI water, the surface tension of IPA is smaller than the surface tension of water in the water tank. This causes the water molecules on the surface of the exposed wafer to be retracted into the water tank and thus achieving the drying purpose.
In the Maragoni drying process, the alcohol vapor frequently cause other processing difficulties. For instance, when residual alcohol is left in the drying tank by depositing on the tank walls, alcohol becomes a source of contamination for the bare silicon surface and causes silicon hole defects. Since alcohol is a necessary element in the Maragoni drying process, it is difficult to completely eliminate residual alcohol on the chamber walls after each batch of wafers is cleaned.
In a normal Maragoni drying process, it has been discovered that the wafer positioned in slot 25 suffers the most severe effect caused by any contaminating conditions in the process. As shown in Table I, Test No. 1 was conducted with the bath filled with DI water, however, no Maragoni drying process was conducted, i.e. the wafers were not exposed to alcohol vapor. It was shown that the defect count is very low on wafers selected from the three different slots of 1, 24 and 25. When the Maragoni drying process was conducted two times, as in Test No. 2, where the bath was empty with no water, the defect counts increases drastically from 6 to 91. A similar Maragoni drying test was conducted in Test No. 3 wherein the Maragoni drying process was carried out only once, the defect count is still very high when compared to data in Test No. 1. Test No. 4 was conducted under conditions similar to that used in Test No. 1, i.e. with no Maragoni drying process being conducted. It is seen that the defect count is very low due to the lack of exposure of the bare wafer surfaces to alcohol, or to the organic vapor of alcohol.
The data shown in Table I suggests that the residual alcohol, or the organic residue after a Maragoni drying process, is the major cause of the high defect count on the wafers. The defects are frequently shown as silicon holes on the bare silicon surface. When the defect count is higher than about 75, the entire wafer is considered as unacceptable for quality reasons. When the defect count is much higher than 75, the wafer may be scrapped. Ideally, the defect count on the bare silicon wafer should be less than 40 for the wafer to pass quality control tests.
It is therefore an object of the present invention to provide a method for drying wafers after wet bench that does not have the drawbacks or shortcomings of the conventional Maragoni drying process.
It is another object of the present invention to provide a method for drying wafers after wet bench that minimizes the amount of organic residue left in the drying apparatus.
It is a further object of the present invention to provide a method for drying wafers after wet bench by reducing the flow rate of alcohol vapor into the drying tank.
It is another further object of the present invention to provide a method for drying wafers after wet bench by reducing the flow rate of alcohol vapor into the drying tank to less than 20 liter/min.
It is still another object of the present invention to provide a method for drying wafers after wet bench by providing an additional fluid conduit into the drying tank for flowing a deionized water stream into the tank and rinsing the tank wall.
It is yet another object of the present invention to provide a method for drying wafers after wet bench by providing fluid conduits into the drying tank for flowing an inert gas into the tank and drying the tank wall after rinsed by DI water.
It is still another further object of the present invention to provide an apparatus for drying wafers after wet bench in an alcohol vapor wherein the apparatus is further equipped with fluid conduits for DI water for rinsing the tank wall and removing organic residues.
It is yet another further object of the present invention to provide an apparatus for drying wafers after wet bench in an alcohol vapor by further providing fluid conduits into the drying tank for flowing an inert gas and drying the tank wall after a rinsing-process for removing the organic residue.
In accordance with the present invention, a method and an apparatus for drying wafers after wet bench without the organic residue problem are provided.
In a preferred embodiment, a method for drying wafers after wet bench can be carried out by the operating steps of first immersing a wafer in a container filled with a volume of deionized water; flowing a mixture of alcohol vapor and inert gas into an upper portion of the container not filled with the volume of deionized water at a flow rate not higher than 20 l/min; and withdrawing the wafer from the deionized water into the upper portion of the container that is filled with the alcohol vapor/inert gas mixture and thereby driving water molecules off the surface of the wafer.
The method for drying wafers after wet bench may further include the step of forming a mixture of alcohol vapor and inert gas from isopropyl alcohol and nitrogen, or the step of flowing the mixture of alcohol vapor and inert gas at a flow rate not higher than 10 l/min; or the step of flowing the mixture of alcohol vapor and inert gas at a flow rate preferably not higher than 6 l/min. The method for drying wafers after wet bench may further include the steps of providing a separate inlet for an inert gas in a sidewall of the container; and flowing an inert gas into the upper portion of the container to further dilute the alcohol vapor/inert gas mixture. The method may further include the step of providing a separate inlet at a location juxtaposed to an alcohol vapor/inert gas mixture inlet through the sidewall of the container. The method may further include the step of flowing the inert gas into the upper portion of the container at a flow rate of at least 5 l/min.
The present invention is further directed to a method for drying wafers after a wet bench process which can be carried out by the operating steps of first filling a container with a volume of deionized water; flowing a mixture of alcohol vapor and inert gas into an upper portion of the container not filled with the volume of deionized water at a flow rate not higher than 20 l/min; flowing a volume of an inert gas into the upper portion of the container at a flow rate of at least 5 l/min; and withdrawing the wafer from the deionized water into the upper portion of the container that is filled with the alcohol vapor/inert gas mixture and thereby driving deionized water molecules off the surface of the wafer.
The method for drying wafers after a wet bench process may further include the step of mounting an inert gas inlet in a sidewall of the container juxtaposed to an inlet for the alcohol vapor/inert gas mixture. The alcohol vapor/inert gas mixture may be formed of isopropyl alcohol vapor and nitrogen. The inert gas may be nitrogen. The method may further include the step of flowing the mixture of alcohol vapor and inert gas at a flow rate not higher than 10 l/min; or at a flow rate preferably not higher than 6 l/min.
The present invention is further directed to an apparatus for drying wafers after wet bench in an alcohol vapor which includes an inner tank formed by four sidewalls and a bottom wall for holding a volume of deionized water therein; a wafer holder for carrying at least one wafer therein and for moving into and out of the volume of deionized water; an outer tank surrounding the inner tank for receiving an overflow from the inner tank; at least one deionized water inlet through a sidewall of the outer tank for flowing deionized water onto and rinsing the sidewalls of the inner tank to remove residual alcohol on at least one of the sidewalls; at least one inert gas inlet through the sidewall of the outer tank for flowing an inert gas onto and drying the at least one of the sidewalls of the inner tank; and an alcohol vapor dispensing means for dispensing an alcohol vapor onto the at least one wafer.
The apparatus for drying wafers after wet bench in an alcohol vapor may further include elevator means for lowering/raising the wafer holder into/out of the volume of deionized water. The at least one deionized water inlet through a sidewall of the outer tank may include four deionized water inlets each for dispensing deionized water onto one sidewall of the inner tank. The at least one inert gas inlet through a sidewall of the outer tank may include four inert gas inlets each for drying one sidewall of the inner tank. The alcohol vapor dispensing means may be an isopropyl alcohol vapor dispensing means.